The present invention relates to the manufacture of flexible intermediate bulk containers (hereinafter bulk bags) for handling flowable materials, and in particular to the manufacture of anti-static films for use in bulk bag liners and to the manufacture of anti-static fabrics for use in bulk bags.
Over the past three decades there has been increasing interest in the use of flexible intermediate bulk containers (hereinafter bulk bags) for handling flowable materials such as chemicals, minerals, fertilizers, foodstuffs, grains and other agricultural products, etc. The advantages resulting from the use of bulk bags include low weight, reduced cost, versatility and, in the case of reusable receptacles, low return freight costs.
Fabrics are often utilized in the construction of various types of bulk containers where strength, flexibility and durability are important. Originally, such containers were fabricated from natural fibers. More recently, however, synthetic fibers manufactured from polypropylene, polyethylene or other polymeric materials have come into almost exclusive use. The popularity of synthetic fibers can be attributed to the fact that they are generally stronger and more durable than their natural fiber counterparts.
Even with the advances in fabric construction resulting from the shift from natural to synthetic fibers, fabrics in general possess qualities that render their use undesirable in certain applications. For example, the friction that occurs as dry flowable materials are handled by fabric receptacles tends to cause a significant build-up and retention of static electric charge within the receptacle. Discharge of the generated static electric build-up is often difficult, if not impossible, to control because fabrics are generally not electrically conductive materials. However, controlled discharge is imperative as static electric potential poses a significant danger of fire or explosion resulting from a static generated electrical spark.
In an effort to address the undesirable static electric discharge characteristic of fabrics, bag manufacturers covered one side of the fabric with a metallic foil-like layer. An adhesive was applied to affix the foil-like layer to the plastic fabric. The foil-like layer was typically comprised of aluminum or some other electrically conductive metal. The foil-covered fabric was then used to construct the receptacle, for example, with the foil side of the fabric comprising the interior surface. The foil layer provided an electrically conductive surface exposed to the flowable materials through which static electricity generated during material handling and was discharged to an appropriate ground.
While adequately discharging static electric build-up if undamaged, the foil-like layer was susceptible to abrasion, tearing and separation from the fabric layer through normal use of the receptacle. For example, in filling, transporting and/or emptying of foil-covered fabric receptacles, abrasion between the flowable material and the foil-like layer tended to cause the foil-like layer to tear and/or separate from the fabric layer. The cumulative effect of such abrasion quickly reduced the effectiveness of the foil-like layer as a static electric discharge surface. Furthermore, tearing of the foil often resulted in a release of foil particles and flakes from the fabric, thereby contaminating the contained flowable materials.
To address the problems experienced with foil-covered fabrics, U.S. Pat. No. 4,833,008, issued to Norwin C. Derby, discloses a metalized fabric comprised of a woven plastic base fabric laminated to a metalized plastic film. The plastic base fabric is preferably a woven polypropylene fabric, and the plastic film is preferably an extruded polypropylene film. The plastic film is metalized through a vapor deposition process whereby a thin film of electrically conductive material is deposited on one side of the plastic film. The woven plastic fabric and the metalized plastic film are then laminated together through use of a plastic adhesive. Unlike foil covered fabrics, the thin conductive layer deposited on the plastic film is not subject to tearing or flaking; however, it is susceptible to chemical reactions.
U.S. Pat. No. 5,244,281, issued to Norwin C. Derby discloses bags made from the fabric disclosed in the Derby ""008 Patent in combination with fabrics impregnated with anti-static compounds. The bags disclosed in the Derby ""281 Patent provide satisfactory anti-static capabilities. Still further developments in the art are disclosed in co-pending Application Ser. No. 8/474,378 filed by Norwin C. Derby and Craig A. Nickell on Jun. 7, 1995 and assigned to the assignee of the present application.
A more recent development in the art of static dissipating bulk bags is the xe2x80x9cDxe2x80x9d type bag which dissipates static electricity without the necessity of grounding by means of corona discharge. At present, xe2x80x9cDxe2x80x9d type bags are manufactured using CROHMIQ(copyright) fabric available from Linq Industrial Fabrics, which is in turn manufactured by weaving into the fabric NEGA-STAT(copyright) fibers manufactured by DuPont. The NEGA-STAT(copyright) fibers, while conductive, do not have sufficient capacitance to produce an energetic spark discharge. Rather, because of small size and geometry, the NEGA-STAT(copyright) fibers facilitate corona discharge. The CROHMIQ(copyright) fabric may also have an extrusion coating of a polymeric material having an anti-static loading of either glycol monosterate or Techmer PM 1530-E4.
xe2x80x9cDxe2x80x9d type bulk bags perform satisfactorily if no liner is required. Heretofore, no liner is available for use in xe2x80x9cDxe2x80x9d type bags. This is limiting because many powders have particle sizes that are too small to be effectively contained by bulk bags not having liners. Also, some regulated materials require a liner for shipment. Thus, a need exists for a bulk bag liner for use with a xe2x80x9cDxe2x80x9d type bag, and for a bulk bag liner that functions similarly to a xe2x80x9cDxe2x80x9d type bag.
Another recent development in the art of static dissipating bulk bags is the xe2x80x9cBxe2x80x9d type bag which is characterized by a breakdown voltage of less than 4,000 volts, thereby eliminating the possibility of propagating brush discharge. A xe2x80x9cBxe2x80x9d type bag is also non-conductive which eliminates the possibility of a spark discharge. Thus, in the use of a xe2x80x9cBxe2x80x9d type bag the only possible discharges of static electricity are brush discharges and corona discharges. xe2x80x9cBxe2x80x9d type bags are therefore adapted for use in almost all environments not involving flammable gases or solvents, and may be used for many sensitive dusts.
xe2x80x9cBxe2x80x9d type bags are less costly to manufacture than xe2x80x9cDxe2x80x9d type bags and are therefore preferable in suitable applications. Like xe2x80x9cDxe2x80x9d type bags, xe2x80x9cBxe2x80x9d type bags perform well when no liner is required. At present, no liner is available for use with xe2x80x9cBxe2x80x9d type bags. Since many bulk bag applications require a liner, a need exists for a liner suitable for use with xe2x80x9cBxe2x80x9d type bags or having the static electricity dissipating characteristics of a xe2x80x9cBxe2x80x9d type bag.
In accordance with a first embodiment of the present invention, an anti-static film suitable for use in manufacturing anti-static liners for bulk bags is fabricated by sandwiching a layer of conductive, low capacitance fibers between two layers of polymeric film. Preferably, the polymeric film layers comprise either polyethylene or polypropylene and may contain either about 6% glycol monosterate or about 10% Techmer PM 1530-E4, whereby the film layers are rendered anti-static. The film layers are preferably provided with small diameter pinholes extending therethrough. The polymeric film-conductive, low capacitance layer assembly is preferably constructed by extrusion coating the inside surface of one of the polymeric film layers with an adhesive comprising the same polymeric material as the film layers, positioning the conductive, low capacitance fibers onto the extruded adhesive, and then compressing the polymeric film layers into engagement with one another, whereby the polymeric film layers are secured one to the other and the conductive, low capacitance layer is secured therebetween by the adhesive.
The foregoing anti-static film may be advantageously employed in the construction of liners for bulk bags. The anti-static film may be cut into a plurality of pieces following a predetermined pattern, and the pieces are then assembled into a bulk bag liner having a desired shape by heat sealing the edges of the pieces one to the other using conventional heat-sealing techniques. In a typical application of the invention, the anti-static bulk bag liner would comprise four side walls, a bottom wall, and a top wall, all heat sealed one to the other along their respective edges. Alternatively, the anti-static film may be fabricated in a double-wide configuration, in which case the resulting bulk bag liner will have side walls comprising two opposed folded corners and two opposed seamed corners. Another alternative is to form the film into a tube having a uniform diameter along its length and gathered at the inlet and outlet of a bulk bag.
In accordance with a first alternative of the first embodiment of the invention, there is provided a length of polymeric film having pin holes formed therein in accordance with a predetermined pattern. A layer of conductive, low capacitance fibers is positioned on the polymeric film. Thereafter, an extruded layer is deposited on top of the layer of conductive, low capacitance fibers. The extruded layer is preferably formed from the same polymeric material that is used in forming the film. It serves to secure the layer of conductive, low capacitance fibers in place on the film, and also serves to fill the pin holes in the film.
In accordance with a second alternative of the first embodiment of the invention, there is provided a length of polymeric film having pin holes formed therein in accordance with a predetermined pattern. A layer of polymeric material is extruded onto one side of the film, and functions to fill the pin holes therein. The extruded layer preferably comprises the same polymeric material that is used in forming the polymeric film. A layer of conductive, low capacitance fibers is not utilized in the practice of the second alternative of the first embodiment of the invention.
The invention further includes a second embodiment comprising an anti-static fabric useful in the construction of bulk bags. The anti-static fabric comprises a layer of conductive, low capacitance fibers secured to a fabric layer. The fabric layer preferably comprises woven polypropylene which may be coated on the exterior surfaces with glycol monosterate or Techmer PM 1530-E4. The fabric layer/conductive, low capacitance layer sandwich is preferably constructed by positioning a layer of conductive, low capacitance fibers on the interior surface of the fabric, and then extrusion coating the interior surface of one of the fabric layers with a layer of adhesive, which preferably comprises the same polymeric materials as is used in the construction of the fabric layers.
The resulting anti-static fabric is used in the construction of bulk bags by cutting the anti-static material into pieces in accordance with a pre-determined pattern. The pieces are then joined together to form a bulk bag having a desired configuration by conventional techniques, typically sewing. The anti-static fabric of the present invention is adapted for use in the construction of bulk bags in accordance with any of the techniques commonly employed in the industry in bulk bag construction.
In accordance with a first alternative of the second embodiment of the invention, there is provided a length of polymeric fabric. A layer of conductive, low capacitance fibers is positioned on the polymeric fabric. Thereafter, an extruded layer is deposited on top of the layer of conductive, low capacitance fibers. The extruded layer is preferably formed from the same polymeric material that is used in forming the fabric. It serves to secure the layer of conductive, low capacitance fibers in place on the fabric.
In accordance with a second alternative of the second embodiment of the invention, there is provided a length of polymeric fabric. A layer of polymeric material is extruded onto one side of the fabric, after which pinholes are formed in the extruded layer. The extruded layer preferably comprises the same polymeric material that is used in forming the polymeric fabric. A layer of conductive, low capacitance fibers is not utilized in the practice of the second alternative of the second embodiment of the invention.